Chess is tactics. More than one strong player has said some variation of this truth. What does that mean? In simple words, it means that if you are not careful you will lose a pawn or a piece.
How do you become careful then? It all starts with board awareness. At all times you should know what is happening on the board. Where the pieces are, how they interact with each other. This awareness should be “on” at all times – don’t “forget” a piece or a pawn just because you’re calculating a sequence of moves on the other side of the board.